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Carbon dioxide (CO2) deservedly gets a lot of bad press because it is responsible for much of the global warming Earth is currently experiencing. Less publicized, but perhaps equally important, CO2 is acidifying oceans, thereby threatening the continued existence of some critical biomes such as coral reefs and kelp forests (acid interferes with the ability of many marine organisms to build their shells). But carbon dioxide also has a kinder, gentler side, as it is an essential resource for plants, and in some cases higher CO2 levels can increase a plant’s ability to carry on photosynthesis. Sean Connell and his colleagues explored this complex personality by studying a marine ecosystem that experiences naturally varying levels of CO2. High CO2 levels and acidity exist near CO2-emitting vents at the study site – a volcanic island (Te Puia o Whakaari) off the coast of New Zealand.

The volcanic Te Puia o Whakaari off the coast of New Zealand’s north island. Credit: James Shook [CC BY 2.5 (https-//creativecommons.org/licenses/by/2.5)], from Wikimedia Commons.

The major players in this ecosystem are the kelp, Ecklonia radiata, several species of turf-forming algae, and two grazers, the snail, Eatoniella mortoni, and the urchin, Evechinus chloroticus. The typical vegetation in the region is a mosaic of kelp forest, some scattered small patches of algal turf, and sea urchin barrens – hard rock without significant vegetation, a result of overgrazing by sea urchins. In contrast, extensive algal mats carpeted the rocks near these vents, and the researchers hypothesized that high CO2 levels caused this shift in dominant vegetation.

Connell and his colleagues chose two vents and two nearby control sites at a depth of 6-8 meters. The CO2 levels and acidification near the vents were approximately equal to the amount projected for the end of the 21stcentury, but there were no differences between vents and controls in temperature, salinity or nutrient concentrations. The researchers estimated photosynthetic rates for kelp and turf algae by measuring the rate of oxygen production. They also estimated snail consumption rates by caging them for 3 days and measuring how much algal turf they removed. They used an analogous approach to measure sea urchin consumption rates.

Conditions at vents had a major impact on both producers and consumers. Kelp production decreased slightly, while turf production increased sharply at vents (Figures A and B below). Urchin density declined (almost to nonexistence) while gastropod density increased markedly at vents (Figures C and D). Lastly, consumption rates (on a per individual basis) by urchins plummeted, while consumption rates by snails increased sharply at vents (Figures E and F).

Comparison of production and consumption at control sites vs. carbon dioxide emitting vents.

These patterns converted the normal mosaic of kelp forest, small algal turf patches and urchin barren into turf-dominated habitats. Algal turf increased in size and frequency near the vents, while kelp forest shrank into near oblivion.

These results can be pictured visually by the graph below. Under conditions of present-day pH and CO2 levels, gross algal production is relatively low and urchin consumption is relatively high, which results in negligible net algal turf production (net production = gross production – urchin and gastropod consumption). High CO2 levels sharply increase gross algal turf production while dramatically decreasing consumption by urchins. Even though gastropod consumption increases slightly at vents, the overall effect on vents is a dramatic increase of net algal turf production. Consequently, the ecosystem experiences regime shift from kelp to algal turf domination.

Summary of effects of CO2 release by vents (bottom) vs Controls (top). Net algal production (red circle) = Gross algal production – urchin and gastropod consumption. Net algal production in dark green zone is predicted to be turf-dominated (as is found near vents), light green is a mosaic, while white zone represents urchin barrens (low production and high consumption). Error bars are 1 standard error.

Under current conditions, kelp is the dominant producer over turf algae in the near offshore ecosystem. High consumption by urchins keep the turf algae in check. But near CO2 emitting vents, high levels of carbon dioxide have a dual effect on this ecosystem, disproportionately increasing turf algae production rate and decreasing urchin abundance and consumption rate. This allows the competitively subordinate turf algae to replace the competitively dominant kelp, resulting in a dramatically changed ecosystem. This occurs in the absence of an increase in ocean temperature. Given that ocean temperature will increase sharply by 2100 (along with CO2 levels), many species interactions are expected to change in the next century, and ecosystem structure and functioning will be very different from what we observe today.